1. Field of the Invention:
The present invention relates to an optical pickup actuator for use in an optical recording/reproducing apparatus. More particularly, the present invention relates to an optical pickup actuator having an aberration correction mechanism and a method of assembling the same.
2. Description of the Related Art:
Recently, technologies for expanding the storage capacities of optical information recording mediums are becoming increasingly important. One method makes the NA (Numerical Aperture) of an objective lens as large as possible in order to increase a recording/reproducing density of a the recording medium. This in turn, reduces the employed wavelength and produces a concentrated beam spot. A concentrated beam spot results because the diameter of the beam spot is inversely proportional to the NA of the objective lens and proportional to the employed wavelength.
Therefore, in an NA specification where the objective lens is increased from about 0.6 to 0.85, the employed wavelength is reduced from about 635 nm to 405 nm. For example, in comparison to the existing DVD (Digital Versatile Disc) a BD (Blue-ray Disc™) has a recording/reproducing density about 5.5 times as large. However, if an optical axis of an objective lens and a surface of a disc are not substantially perpendicular to one another, comatic aberration may result. The amount of such an aberration is proportional to the thickness of the objective lens and the cube of the NA of the objective lens. Therefore, the thickness of a light transmissive layer in such a BD is reduced from about 0.6 mm (of the existing DVD) to 0.1 mm to reduce the amount of aberration of the beam spot. Consequently, the above-mentioned problems are addressed.
However, because the amount of spherical aberration is also proportional to the fourth power of the NA of an objective lens and a deviation in thickness of a record medium, the recording medium preferably has a thickness deviation within about ±3 μm in order to employ an objective lens having an NA in the order of about 0.85. However, it is relatively difficult to manufacture a recording medium within the above-mentioned range of thickness deviation.
Therefore, in order to employ an objective lens having a high NA in the order of about 0.85, it is preferable to compensate for the spherical aberration which results due to the recording medium thickness deviation. For example, unexamined Japanese Patent Publication Nos. 2000-030281, 2001-143303 and 2003-006902, the entire disclosures of which are hereby incorporated by reference in their entirety, disclose technologies which correct spherical aberrations which occur via a deviation in thickness of a liquid crystal recording medium.
For example, Patent Publication No. 2003-006902 discloses an optical pickup mounted on an actuator moving unit (hereinbelow, referred to as “blade”). The optical pickup is fitted with an objective lens and a liquid crystal device for correcting aberration in unison. Therefore, problems with optical performance deterioration, made worse by a central misalignment between a liquid crystal device and an objective lens, can be avoided.
The optical pickup disclosed in unexamined Patent Publication No. 2003-006902 addressed problems with respect to optical performance deterioration caused by the two optical components by mounting an objective lens along with a liquid crystal device for correcting aberration on the blade. However, the problem remains in that a central deviation may occur between the objective lens and the liquid crystal lens in the course of assembling the objective lens and the liquid crystal device to the blade, which is an injection-molded product.
As a result of investigating the reproducing performance in connection with a positional tolerance of the liquid crystal device for correcting aberration, it has been proposed that about 1% deterioration in jitter value is caused when a deviation in the range of about 25 to 30 nm (the central deviation between the liquid crystal device and the objective lens) occurs. Experiments have found that a deviation of about 0.1 mm may occur while the objective lens and the liquid crystal device are assembled to the injection-molded blade.
Moreover, if a liquid crystal device is fitted in a blade as described above, wiring of lead wires or the arrangement of a flexible substrate for applying voltage to the liquid crystal device may be more complicated.
Accordingly, there is a need for an improved optical pickup actuator which prevents deterioration in optical performance caused by the deviation of the two optical components, and a method of assembling the same.